Transcriptomic Identification and Biochemical Characterization of HmpA, a Nitric Oxide Dioxygenase, Essential for Pathogenesis of .

Nitric oxide (NO) and its derivatives are important effectors of host innate immunity, disrupting cellular function of infecting pathogens. Transcriptome analysis of , an opportunistic human pathogen, identified a set of genes induced upon exposure to NO. Among them, (), encoding a multidomain NO dioxygenase, was the most greatly induced upon exposure to NO and was thus further characterized. Absorption spectra demonstrated that HmpA is a heme protein in which the heme iron can exist in either reduced, NO-bound, or oxidized state. Biochemical studies revealed that HmpA is a flavohemoglobin containing equimolar amounts of heme and FAD as cofactors. The and values of HmpA for NO at 37°C, the temperature encountered by in the host, were greater than those at 30°C, indicating that HmpA detoxifies high levels of NO effectively during infection. Compared with the wild type, the mutant exhibited a lower NO-decomposition activity and impaired growth in the presence of NO . Also, the cytotoxicity and survival of the mutant infecting the NO-producing murine macrophage cells were lower than those of the wild type. Furthermore, the mouse lethality of the mutant was reduced compared to that of the parental wild type. The combined results revealed that HmpA is a potent virulence factor that is induced upon exposure to NO and important for the survival and pathogenesis of during infection.